PHYLUM ECHINODERMATA 



407 



of muscular fibres. Concealed from view by the centro-dorsal 

 ossicle is a thin plate termed the " rosette " (ros), formed by the 

 coalescence of the hascds of the larva. At the sides are five 

 first radial ossicles (B^), also concealed by the centro-dorsal 

 ossicle : with each of these articulates a second radial (B^), which 

 is visible beyond the centro-dorsal. With each of the second 

 radials articulate two third radials (E'^), each forming the base of 

 the corresponding arm-branch. 



The ossicles of the arms — brachials {Br}, Br}) — are arranged 



in a single row in each arm. 

 in the direction of the long 

 axis of the arm, strongly con- 

 vex on their aboral surfaces, 

 longitudinally grooved on the 

 oral surface, and connected to- 

 gether by the investing epi- 

 dermis and by bundles of 

 muscular fibres, by the con- 

 tractions of which the move- 

 ments of the arms are brought 

 about. Fringing the sides of 

 each arm are two rows of side- 

 branches, or pinmdes, each sup- 

 ported by its row of connected 

 ossicles, and each grooved along 

 its oral surface. 



The ccelome contains num- 

 erous strands of connective- 

 tissue which serve to suspend 

 the various organs. 



Extending through the arms 

 and pinnules between the sup- 

 porting ossicles and the am- 

 bulacral grooves are three 

 canals which are prolongations 

 of the ccelome (Fig. 333, cal. 

 terdacular canals 



They are somewhat elongated 



md. ainb 

 coet catt 



Fig. 333. — Antedonj transverse section of a 

 pinnule, a-wh. ne. radial nerve of the super- 

 ficial (ambulacral) nervous system ; rt.r. ne. 

 axial nerve ; coil. can. sub-tentacular and 

 coiliac canals ; mus. muscles ; neur. ves. radial 

 sinus of the perihaemal system ; rad. amb. 

 radial ambulacral vessel giving off branches 

 to the tentacles. Between the paired sub- 

 tentacular and unpaired cosliac canals is the 

 genital rachis. The small round bodies above 

 the line from rad. amb. are the succuli. (After 

 Teuscher.) 



can). Two of these — the sub- 

 form a pair separated from one another by a 

 median septum underlying the ambulacral groove. The other — 

 the cceliac canal — runs between these and the supporting ossicles 

 (oss). The sub-tentacular canals and the coeliac canal communicate 

 with one another at the extremity of each arm. 



The enteric canal begins with a wide, funnel-shaped ceso- 

 phcujus leading to a spacious stomueh which gives off a number of 

 short, blunt diverticula and a pair of longer, narrower, "hepatic " 

 cteca, which are slightly branched at the ends. Distally the stomach 

 becomes contracted and opens into a wide intestine, which winds 

 round the ccelome, becoming narrower where it passes upwards to 



